Signal processing devices, in particular radiofrequency transceivers, are used inter alia in mobile communication devices which have only a limited supply of energy, usually in the form of a battery or a rechargeable battery. Therefore, the requirement exists for a low current consumption of such transceivers, in order to increase the so-called service life of mobile communication devices. In the case of radiofrequency transceivers, the current consumption is particularly dependent on the operating point of the individual switching components of a transceiver. In this case, the term operating point defines that region of the family of characteristic curves of the circuit component which is used in operation of the circuit component. The operating point is determined by a supply voltage, a supply current or a programming voltage.
In addition to the current consumption, the operating point also influences the system properties of the transceiver. Thus, by way of example, the operating point of the circuit components of a receiver front end of a transceiver influences the noise figure and also the signal/noise ratio of a received and partly processed signal. The operating point of a receiver amplifier in a transceiver can alter the linearity of a reception amplifier and thus lead to distortions.
Linearity and signal/noise ratio are of particular importance for the data transmission quality.
Operating points in circuit components of the transmitter influence the transfer response during the signal processing and thus the output power and also the frequency spectrum of the output signal.
In order to ensure error-free operation even under unfavorable conditions, for example in the case of small input signals and high external temperatures, it is necessary to choose the operating points suitably for such conditions. As a result, under favorable conditions more current is consumed than would be necessary to achieve the required properties with regard to the signal quality.
In modern radiofrequency transceivers and signal processing devices, the operating points of individual circuit components are therefore programmed suitably or predefined for a critical case. One example of a programming of a radiofrequency transceiver via the baseband unit—connected in the transceiver—by means of a three-conductor bus is shown by the document U.S. Pat. No. 5,994,931. Via this interface, by way of example, the operating point of the input amplifiers of the transceiver is set and the current consumption is thus reduced depending on input signals. The signals transmitted via the interface are generated by a control circuit in this case. For this purpose, however, it is necessary to assess signals representing states of the transceiver. In the context of the increasing integration density and the desire to integrate as many switching components as possible in a semiconductor body, the construction with a three-conductor bus and external control circuits means additional space requirement and costs.